The present invention relates to a motor-driven disk brake for generating braking force by using a torque obtained from a rotary actuator. More particularly, the present invention relates to a motor-driven disk brake having a brake releasing function capable of automatically releasing the brake from a lock-up state caused by a failure of the rotary actuator.
A conventional motor-driven disk brake has a caliper containing a piston, a rotary actuator and a motion converting mechanism for transmitting rotation of the rotary actuator to the piston after converting it into rectilinear motion. The piston is driven in response to rotation of the rotary actuator to press a brake pad against a disk rotor, thereby generating braking force in proportion to the torque of the rotary actuator. In this type of motor-driven disk brake, if the rotary actuator fails owing, for example, to disconnection of the power supply line during braking when braking thrust is being generated in the piston, the thrust remains in the piston because of the internal resistance of the rotary actuator and the internal resistance of a speed-reducing gear if it is provided to increase the rotational torque of the rotary actuator. Consequently, it becomes difficult to release the brake.
Under these circumstances, some schemes have heretofore been devised to automatically release the brake in the event of a failure in the rotary actuator. For example, International Publication WO 00/60255 (hereinafter referred to as “patent document 1” discloses that the, rotor of a motor is provided with a spring to rotate the rotor in reverse by the return force of the spring. Published Japanese Translation of PCT International Publication No. 2000-507333 (corresponding U.S. Pat. No. 6,179,097: hereinafter referred to as “patent document 2”) discloses that an electric motor for returning the piston is incorporated in a part of a speed-reducing mechanism attached to the motor, and the motor for returning the piston is activated in the event of a failure in the main electric motor for advancing the piston. Published Japanese Translation of PCT International Publication No. 2001-506348 (corresponding U.S. Pat. No. 6,257,377; hereinafter referred to as, “patent document 3”) discloses that an electromagnetic clutch is provided in a thrust load bearing part for bearing the nut of the motion converting mechanism, and in the event of a motor failure, the electromagnetic clutch is released to retract the nut.
However, with the scheme disclosed in the patent document 1, the amount of deflection of the spring increases with the rotation of the rotor. Therefore, the rotor has to be rotated against a large spring force for each braking operation. Because the amount of rotation of the rotor increases in accordance with the degree of wear of the brake pad, the consumption of electric power for rotating the rotor increases with age. In addition, because the return force of the spring increases in proportion to the wear of the brake pad, a stable operation cannot be obtained.
The scheme disclosed in the patent document 2 requires a motor for releasing the brake in addition to the main motor for advancing the piston. Therefore, not only the caliper unavoidably increases in size, but also the production cost increases. Moreover, there is also possibility that the brake releasing motor itself may fail. Therefore, the brake lacks reliability.
The scheme disclosed in the patent document 3 suffers from problems such as an increase in power consumption due to the supply of power to the electromagnetic clutch and an increase in cost due to the provision of the electromagnetic clutch.
The present invention was made in view of the above-described problems with the prior art.
An object of the present invention is to provide a highly reliable motor-driven disk brake made capable of being released mechanically in the event of a motor failure without causing an increase in power consumption and a large increase in production cost.